1. Field of the Invention
This invention relates to the field of precision inductors, particularly tunable inductors that can be integrated.
2. Description of the Related Art
Demands for more efficient, higher performance, smaller electronic circuits are regularly heard. Integration typically results in smaller circuits, but often at the expense of performance. In the area of wireless communications, for example, highly integrated transceiver designs often require high performance passive components--particularly inductors and capacitors--but present integrated versions of these components typically possess several performance-degrading shortcomings.
It is often desirable for the inductors in an RF transceiver to have low loss and high quality factor (Q) characteristics: high-Q tank circuits in voltage-controlled oscillators (VCOs) reduce phase noise, low loss inductors in low noise amplifiers (LNAs) and power amplifiers improve input and output matching, and inductively loaded circuits in general offer significant power savings when implemented with high-Q components. These demands are not easily met, however, when the passive components are integrated. For example, the frequency selectivity of a circuit in which an inductor is used increases with its Q, requiring extremely precise values of inductance which are difficult to attain in an integrated device. Deviations from the desired inductance value must be compensated for with other frequency-variable passive elements, which may be limited in both Q and tuning range.
The performance of integrated inductors is also limited by the parasitic capacitance inherently present between coil and substrate. This capacitance resonates with the inductance at the self-resonant frequency, which often serves as a frequency limit below which circuits must operate. Larger coils are needed to obtain larger values of inductance, but the coil's larger area results in higher values of capacitance and a lower self-resonant frequency. Larger coils also possess a higher series resistance, increasing losses and reducing Q.
At present, these shortcomings are best addressed with fixed value off-chip inductors, but variations in component value, assembly technology, die interconnection methods, and packaging variations can cause changes in the inductance seen by the design. Using off-chip components also increases assembly time and reduces reliability. Also, the power dissipation of buffers which drive off-chip signals can be significant, resulting in higher power consumption.
"Active" inductor circuits, based on transistor gain stages and gyrator configurations, for example, have also been used to achieve integrated inductances. These circuits typically involve the use of active devices in a feedback loop, which gives rise to a number of problems: the active devices can generate unacceptably high levels of noise, they can saturate--limiting their dynamic range, and they consume amounts of power that may be undesirable in low power systems.